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From: oliver@karakorum.berkeley.edu (Oliver Sharp)
Newsgroups: comp.compilers
Subject: Defining polymorphism vs. overloading
Keywords: design
Message-ID: <9008310419.AA06194@karakorum.berkeley.edu>
Date: 31 Aug 90 04:19:43 GMT
Sender: compilers-sender@esegue.segue.boston.ma.us
Reply-To: oliver@karakorum.berkeley.edu (Oliver Sharp)
Organization: Compilers Central
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Approved: compilers@esegue.segue.boston.ma.us

Once upon a time, shortly before an exam in a programming language course, 
I asked myself "exactly what is the difference between these two things?"  
The answer being "Ummm ... well ...", I decided to poke around.  The
result is that things seem a bit murky.  It seems as though:

  o  Overloading means using the same name (or symbol) to invoke different
    code depending on the types of the arguments (or operands).  So, an example
    is +, which may do radically different things for integers, floats, and
    complex numbers.  In other words, you have different chunks of code
    depending on the type.  You may need to do some implicit casting, as in
    promoting an integer to a complex number, before you can actually perform
    the operation.

  o  Polymorphism means using the same piece of code to operate on objects of
    different types.  In LISP, cons is polymorphic because you can give it
    arguments of different types but it goes ahead and does its thing without
    worrying about it.

Well, that seems pretty straight-forward so far, but in practice the
distinction isn't so clean.  For example, think about square braces in C.
I've heard people say that they are overloaded, because they can apply to
arrays of different types ... but, by our just agreed on definitions, they
are really polymorphic.  I'd look at someone funny if they said "braces are
polymorphic in C", though, wouldn't you?  I've also heard people describe
things that are clearly overloaded as being polymorphic.

Having just taken a Linguistics class from Lakoff, I could explain all this 
away with a lot of learned talk about radial categories, and prototypical
cases, and so forth ... but I'm really curious.  What do YOU think the
distinction is?  Or isn't there one?  Do you use these terms consistently?

- Oliver Sharp
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